1. Field of the Invention
The present invention relates to a sheet conveying apparatus which conveys a sheet, and an image forming apparatus, such as a copying machine, a printer, or a facsimile machine, which includes the sheet conveying apparatus.
2. Description of the Related Art
In the past, an image forming apparatus using an electrophotographic system generates a visible image by developing a latent image formed on a photosensitive drum as an image bearing member and transfers the visible image (toner image) to a sheet by using an electrostatic force. Then, the toner image on the sheet is fixed by heat and pressure. In this manner, an image is recorded and formed on the sheet.
As a fixing apparatus of such an image forming apparatus, a heat roller fixing method is employed. Specifically, a fixing nip portion is formed by pressing an elastic pressure roller against a fixing roller which is maintained at a predetermined temperature by a heat source such as a heater provided thereinside, and the toner image is fixed to the sheet in the fixing nip portion.
However, in the heat-fixing process of the fixing apparatus, since heat and pressure is added to the sheet on which the toner image is transferred, moisture is evaporated from the inside of the sheet after a pressure nip portion and a pressure nip. Due to a change in an amount of moisture by the heat of the sheet generated at this time and a stress by the pressure applied to the sheet, there occurs a so-called curl phenomenon that the sheet is curved and a so-called wave phenomenon that the sheet is undulated.
Here, a sheet-shaped paper, which is most commonly used as the sheet, is considered as a fiber level. The sheet is configured by entangling short fibers with one another, and moisture is contained inside the fibers or between the fibers. Furthermore, since the fiber and the moisture have an equilibrium state in a state in which hydrogen bonds are generated, the fiber and the moisture maintain smoothness.
However, when the heat and the pressure are applied to the sheet in the fixing process, misalignment occurs in the fibers due to the pressure. When the heat is applied in that state and the moisture is evaporated, more hydrogen bonds are generated in the fibers, causing deformation. When the sheet is left as it is, the sheet absorbs moisture from the environment and the hydrogen bonds of the fibers are disconnected and try to return to the original state. However, moisture does not enter between some fibers of the sheet and thus the deformation of the sheet is maintained. As a pattern of the deformation, there are the curl and the wave described above. The curl is generated by a difference of expansion and contraction in the front and back sides of the sheet, and the wave is generated by a difference of expansion and contraction in the center portion and the edge portion of the sheet.
Therefore, a configuration that imparts water to a sheet by passing the sheet through a nip portion of a pair of humidifying rollers imparting water is disclosed (see U.S. Patent Application Publication No. 2008/0089728 A1 and U.S. Patent Application Publication No. 2009/0245908 A1). The hydrogen bonds between the fibers of the sheet are disconnected once by imparting water to the sheet having passed through the fixing apparatus by the pair of humidifying rollers, making it easy to correct the curl of the sheet or the conveying-direction length (expansion difference) of the width-direction center portion and edge portion.
However, when a toner image of a relatively high density is formed on the surface of the sheet, the toner image becomes a barrier and the imparted water hardly penetrates into the sheet. Therefore, the imparted water becomes water droplets and is attached to the surface of the toner image. Hence, the conveying function at more downstream than the pair of humidifying rollers may be hindered and the image also may be adversely affected.
Also, in the case of a coated sheet having a coating layer such as calcium carbonate or porous silica, the coating layer becomes an additional barrier and the imparted water hardly penetrates into the sheet.
That is, when the sheet, to which the water droplets are attached, is nipped at the nip of the conveying roller of the downstream, the water droplets are transferred on and attached to the conveying surface of the conveying roller. Furthermore, when the conveying roller, to which the water droplets are attached, nips a subsequent sheet, the water droplets are transferred on and attached on the surface of the sheet. A conveying force between the conveying roller and the sheet, to which the water droplets are attached, is lowered and becomes unstable, causing a conveyance failure. It is considered that similar phenomenon occurs between conveying guides as well as the conveying roller.
Also, the water droplets attached to the toner image on the surface of the sheet enters between the toner mass (toner image) and the sheet fiber surface directly under the toner mass (toner image), and that portion is swollen to have a blister. In addition, the water droplets are evaporated on the toner image and spot-shaped marks are left, causing image deterioration.